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  <title>P0528r0: The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
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 * Structural Markup
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 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
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 * Code
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 * Special Sections
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 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
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 * Definition Boxes
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 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
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 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
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 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
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 *   - .copyright for the copyright
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 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
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 *
 ******************************************************************************/

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/******************************************************************************/

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/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

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/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

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/******************************************************************************/

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/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
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/** Change Marking ************************************************************/

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/** Miscellaneous improvements to inline formatting ***************************/

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/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

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/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
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/** General Hyperlinks ********************************************************/

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	/* Backout above styling for W3C logo */
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/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

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/******************************************************************************/
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/******************************************************************************/

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/** Assertion Box *************************************************************/
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/** Advisement Box ************************************************************/
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/** Spec Obsoletion Notice ****************************************************/
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/** Entity Definition Boxes ***************************************************/

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		margin: 1.2em 0;
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/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

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		width: 4em;
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/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

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  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P0528r0<br>The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2016-11-12">12 November 2016</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0528r0">http://wg21.link/P0528r0</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jfbastien@apple.com">JF Bastien</a> (<span class="p-org org">Apple</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:bigcheesegs@gmail.com">Michael Spencer</a> (<span class="p-org org">Sony Playstation</span>)
     <dt>Audience:
     <dd>SG1, LEWG, LWG, CWG
     <dt>Project:
     <dd><span>ISO JTC1/SC22/WG21: Programming Language C++</span>
     <dt>Source:
     <dd><span><a href="https://github.com/jfbastien/papers/blob/master/source/P0528r0.bs">github.com/jfbastien/papers/blob/master/source/P0528r0.bs</a></span>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
  <div class="p-summary" data-fill-with="abstract">
   <p>I was thinking how nothing lasts—especially a struct’s padding bits—and what a shame that is when used with atomic compare-and-exchange.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#story"><span class="secno">1</span> <span class="content">Story Time</span></a>
     <ol class="toc">
      <li><a href="#struct"><span class="secno">1.1</span> <span class="content">The Curious Case of <code class="highlight"><span></span><span class="k">struct</span></code></span></a>
      <li><a href="#union"><span class="secno">1.2</span> <span class="content">A <code class="highlight"><span></span><span class="k">union</span></code> Born in Unusual Circumstances</span></a>
      <li><a href="#fp"><span class="secno">1.3</span> <span class="content">Points are Floating, Even <code class="highlight"><span></span><span class="n">NaN</span></code>wards</span></a>
      <li><a href="#store"><span class="secno">1.4</span> <span class="content">You Never Know What’s Storing</span></a>
      <li><a href="#miss"><span class="secno">1.5</span> <span class="content">Defining the Opportunities We’ll Miss</span></a>
      <li><a href="#impression"><span class="secno">1.6</span> <span class="content">Making the Greatest Impression</span></a>
      <li><a href="#life"><span class="secno">1.7</span> <span class="content">Standards Can Only Be Understood Backward, they Must Be Lived Forward</span></a>
     </ol>
    <li>
     <a href="#word"><span class="secno">2</span> <span class="content">Proposed Wording</span></a>
     <ol class="toc">
      <li><a href="#word-meta-type-synop"><span class="secno">2.1</span> <span class="content">Header <code class="highlight"><span></span><span class="o">&lt;</span><span class="n">type_traits</span><span class="o">></span></code> synopsis [<strong>meta.type.synop</strong>]</span></a>
      <li><a href="#word-mta-unary-prop"><span class="secno">2.2</span> <span class="content">Type properties [<strong>meta.unary.prop</strong>]</span></a>
      <li><a href="#word-atomics-types-operations-req"><span class="secno">2.3</span> <span class="content">Requirements for operations on atomic types [<strong>atomics.types.operations.req</strong>]</span></a>
      <li><a href="#word-atomics-types-generic"><span class="secno">2.4</span> <span class="content">Atomic types [<strong>atomics.types.generic</strong>]</span></a>
      <li><a href="#word-atomics.syn"><span class="secno">2.5</span> <span class="content">Header <code class="highlight"><span></span><span class="o">&lt;</span><span class="n">atomic</span><span class="o">></span></code> synopsis [<strong>atomics.syn</strong>]</span></a>
     </ol>
    <li><a href="#sample"><span class="secno">3</span> <span class="content">Sample Program</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <p>This issue has been discussed by the authors at every recent Standards meetings,
yet a full solution has been elusive despite helpful proposals. We believe that
this proposal can fix this oft-encountered problem once and for all.</p>
   <h2 class="heading settled" data-level="1" id="story"><span class="secno">1. </span><span class="content">Story Time</span><a class="self-link" href="#story"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="struct"><span class="secno">1.1. </span><span class="content">The Curious Case of <code class="highlight"><span></span><span class="k">struct</span></code></span><a class="self-link" href="#struct"></a></h3>
   <p>Using the Standard’s atomic compare-and-exchange on <code class="highlight"><span></span><span class="k">struct</span></code>s with padding bits
is fraught with peril: the <code class="highlight"><span></span><span class="n">desired</span></code> value is passed by value to all of the
methods, which doesn’t guarantee than any particular bit pattern is preserved in
the bits which don’t participate in the <em>value representation</em>. the
padding bits mean that there are multiple <em>object representations</em> for
the same value representation.</p>
   <p>Indeed:</p>
   <ul>
    <li data-md="">
     <p>Object initialization doesn’t guarantee any particular value for padding bits.</p>
    <li data-md="">
     <p>Copying an object can change its padding bits, on <strong>every copy</strong>.</p>
   </ul>
   <p>Whereas using compare-and-exchange acts on the <strong>entire</strong> object,
padding bits included. From 29.6.5 Requirements for operations on atomic types
[<strong>atomics.types.operations.req</strong>], ¶24:</p>
   <blockquote>
     [ <em>Note:</em> For example, the effect of <code class="highlight"><span></span><span class="n">atomic_compare_exchange_strong</span></code> is 
<pre class="highlight"><span></span><span class="k">if</span> <span class="p">(</span><span class="n">memcmp</span><span class="p">(</span><span class="n">object</span><span class="p">,</span> <span class="n">expected</span><span class="p">,</span> <span class="k">sizeof</span><span class="p">(</span><span class="o">*</span><span class="n">object</span><span class="p">))</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span>
    <span class="n">memcpy</span><span class="p">(</span><span class="n">object</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">desired</span><span class="p">,</span> <span class="k">sizeof</span><span class="p">(</span><span class="o">*</span><span class="n">object</span><span class="p">));</span>
<span class="k">else</span>
    <span class="nf">memcpy</span><span class="p">(</span><span class="n">expected</span><span class="p">,</span> <span class="n">object</span><span class="p">,</span> <span class="k">sizeof</span><span class="p">(</span><span class="o">*</span><span class="n">object</span><span class="p">));</span></pre>
    <p><em>— end note</em> ]</p>
   </blockquote>
   <p>Compare-and-exchange is specified this way because that’s how hardware works: <code class="highlight"><span></span><span class="n">cmpxchg</span></code> or load-linked / store-conditional instuctions must operate on the
entire address range to be atomic.</p>
   <p>The interactions between non-atomic and atomic objects which contain padding
bits means that compare-and-exchange is <strong>never</strong> guaranteed to
succeed, even if the program only ever uses a single value representation.</p>
   <p>We’ve included a <a href="#sample">§3 Sample Program</a> which exhibits this behavior.</p>
   <h3 class="heading settled" data-level="1.2" id="union"><span class="secno">1.2. </span><span class="content">A <code class="highlight"><span></span><span class="k">union</span></code> Born in Unusual Circumstances</span><a class="self-link" href="#union"></a></h3>
   <p>Unions can exhibit a similar problem: they can contain padding, but can also
contain bits which don’t participate in their active member’s value
representation.</p>
   <h3 class="heading settled" data-level="1.3" id="fp"><span class="secno">1.3. </span><span class="content">Points are Floating, Even <code class="highlight"><span></span><span class="n">NaN</span></code>wards</span><a class="self-link" href="#fp"></a></h3>
   <p>Astute readers will note that value representations with multiple object
representations exist for more than simply <code class="highlight"><span></span><span class="k">struct</span></code> and <code class="highlight"><span></span><span class="k">union</span></code>. Even with <code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span></code>'s requirement that objects be trivially copyable, it is possible
for boolean, signed integral and floating-point values to suffer from this
problem.</p>
   <p>The common example where this happens is when signaling <code class="highlight"><span></span><span class="n">NaN</span></code>s are present: some
platforms canonicalize them to quiet a <code class="highlight"><span></span><span class="n">NaN</span></code>. This is a similar problem to
padding bits: copying can change the object representation.</p>
   <h3 class="heading settled" data-level="1.4" id="store"><span class="secno">1.4. </span><span class="content">You Never Know What’s Storing</span><a class="self-link" href="#store"></a></h3>
   <p>Another surprising case is when atomic <code class="highlight"><span></span><span class="n">store</span></code> or <code class="highlight"><span></span><span class="n">exchange</span></code> operations are
implemented using compare-and-exchange. This happens when the target
architecture doesn’t have a suitably-sized <code class="highlight"><span></span><span class="n">store</span></code> or <code class="highlight"><span></span><span class="n">exchange</span></code> instructions,
but has an appropriate <code class="highlight"><span></span><span class="n">cmpxchg</span></code> or load-linked / store-conditional. To
guarantee lock-freedom, implementations rely on compare-and-exchange. This is
transparent to the developer, but can lead to permanent failure to <code class="highlight"><span></span><span class="n">store</span></code> or <code class="highlight"><span></span><span class="n">exchange</span></code> if padding bits are present.</p>
   <p>The same implementation approach is possible for <code class="highlight"><span></span><span class="n">load</span></code>s, but failure to perform
an idempotent compare-and-exchange is the same as a <code class="highlight"><span></span><span class="n">load</span></code>, regardless of
padding bits since it yields the previous value.</p>
   <h3 class="heading settled" data-level="1.5" id="miss"><span class="secno">1.5. </span><span class="content">Defining the Opportunities We’ll Miss</span><a class="self-link" href="#miss"></a></h3>
   <p>This problem is valid for <code class="highlight"><span></span><span class="k">union</span></code>, some scalar representations, and some <code class="highlight"><span></span><span class="n">store</span></code> / <code class="highlight"><span></span><span class="n">exchange</span></code> implementations. The authors nonetheless believe that it is a
separable from that of padding bits on <code class="highlight"><span></span><span class="k">struct</span></code>s. Indeed, all <code class="highlight"><span></span><span class="k">struct</span></code> with
padding bits exhibit this problem, whereas only limited scalar values are
affected. The same applies for <code class="highlight"><span></span><span class="k">union</span></code>: the issue can only surface when the
widest member isn’t the active member (though <code class="highlight"><span></span><span class="k">union</span></code>s do suffer of the padding
problem). Only some <code class="highlight"><span></span><span class="n">store</span></code> / <code class="highlight"><span></span><span class="n">exchange</span></code> implementations are affected, in which
case their <code class="highlight"><span></span><span class="k">struct</span></code>s with padding bits are also affected. We don’t believe <code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="kt">bool</span><span class="o">></span></code> suffers from this problem.</p>
   <p>Put another way: compare-and-exchange of a <code class="highlight"><span></span><span class="k">struct</span></code> with padding bits is <strong>always</strong> wrong, whereas the other similar issues are <strong>sometimes</strong> wrong.</p>
   <p>This leads us to the following conclusions:</p>
   <ol>
    <li data-md="">
     <p>For types with padding bits a simple solution exists using the existing
 Standard: developers should explicitly define padding members if they
 intend to compare-and-exchange a type. Were an approach such as <a data-link-type="biblio" href="#biblio-n3986">[N3986]</a> adopted, developers could rely on this facility to avoid padding bits more
 easily.</p>
    <li data-md="">
     <p>Other types which don’t have unique object representations merit another
 solution: developers should never compare-and-exchange a value which has
 multiple object representations. All other values of that type can be
 compare-and-exchanged without issue.</p>
    <li data-md="">
     <p>Implementations which rely on compare-and-exchange for the <code class="highlight"><span></span><span class="n">store</span></code> and <code class="highlight"><span></span><span class="n">exchange</span></code> functions of certain instantiations of <code class="highlight"><span></span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> could apply a
 similar solution to that proposed in this paper. The authors are interested
 in writing a separate paper to address the issue.</p>
   </ol>
   <p>This paper only addresses the first conclusion.</p>
   <h3 class="heading settled" data-level="1.6" id="impression"><span class="secno">1.6. </span><span class="content">Making the Greatest Impression</span><a class="self-link" href="#impression"></a></h3>
   <p>Conclusion 1. above is addressable by developers, but the authors believe that
this Standard should prevent this dangerous usage of compare-and-exchange. This
is better handled as a requirement of the methods than as a QoI warning.</p>
   <p>The authors therefore propose using concepts' <code class="highlight"><span></span><span class="n">requires</span></code> clause to all the
compare-and-exchange functions, forbidding their usage on a type which has
padding bits. All other <code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span></code> operations would remain valid, only
compare-and-exchange would become a compilation error if used.</p>
   <p>The Standard, if <a data-link-type="biblio" href="#biblio-p0020r3">[P0020r3]</a> is adopted, will specify 4 specializations for <code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span></code>:</p>
   <ul>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code></p>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">*></span></code></p>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">integral</span><span class="o">></span></code></p>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">floating</span><span class="o">-</span><span class="n">point</span><span class="o">></span></code></p>
   </ul>
   <p>Since <a data-link-type="biblio" href="#biblio-p0258r2">[P0258r2]</a> the Standard contains the type trait <code class="highlight"><span></span><span class="n">has_unique_object_representations</span></code>, as defined in 20.15.4.3 Type properties
[<strong>meta.unary.prop</strong>] ¶9. This trait initially seems ideally suited
to our purpose:</p>
   <blockquote>
    <p>The predicate condition for a template specialization <code class="highlight"><span></span><span class="n">has_unique_object_representations</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value</span></code> shall be satisfied if and only
  if:</p>
    <ul>
     <li data-md="">
      <p>T is trivially copyable, and</p>
     <li data-md="">
      <p>any two objects of type <code class="highlight"><span></span><span class="n">T</span></code> with the same value have the same object
representation, where two objects of array or non-union class type are
considered to have the same value if their respective sequences of direct
subobjects have the same values, and two objects of union type are
considered to have the same value if they have the same active member and
the corresponding members have the same value.</p>
    </ul>
    <p>The set of scalar types for which this condition holds is
  implementation-defined. [ <em>Note:</em> If a type has padding bits, the
  condition does not hold; otherwise, the condition holds true for unsigned
  integral types. <em>— end note</em> ]</p>
   </blockquote>
   <p>We could apply it to the <code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> variants of compare-and-exchange only,
avoiding the pointer, integral and floating-point specializations. This
unfortunately leaves out what seems like a useful case:</p>
<pre class="highlight"><span></span><span class="k">struct</span> <span class="n">T</span> <span class="p">{</span>
  <span class="kt">float</span> <span class="n">a</span><span class="p">;</span>
  <span class="kt">float</span> <span class="n">b</span><span class="p">;</span>
<span class="p">};</span>
<span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="n">t</span><span class="p">;</span>
<span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">has_unique_object_representations_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="p">);</span></pre>
   <p>This type typically has no padding bits, fits the <code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> specialization, yet on some platforms does <strong>not</strong> have a unique
object representation because of its floating-point members. The above type is
often spelled as <code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">complex</span><span class="o">&lt;</span><span class="kt">float</span><span class="o">>></span></code>, which is a very useful
type, even atomically. Naïvely adding <code class="highlight"><span></span><span class="n">requires</span> <span class="n">has_unique_object_representations_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> on the compare-and-exchange members of <code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> would—among other things—forbid using compare-and-exchange on
complex numbers. The authors believe this is unacceptable.</p>
   <h3 class="heading settled" data-level="1.7" id="life"><span class="secno">1.7. </span><span class="content">Standards Can Only Be Understood Backward, they Must Be Lived Forward</span><a class="self-link" href="#life"></a></h3>
   <p>We find ourselves unable to use <code class="highlight"><span></span><span class="n">has_unique_object_representations</span></code>, which was
intended for future <code class="highlight"><span></span><span class="n">std</span><span class="o">::</span><span class="n">hash</span></code> proposals but was hoped to also be usable for
our present usecase. We thus propose a new type trait, tentatively named <code class="highlight"><span></span><span class="n">has_padding_bits</span></code>.</p>
   <p>If this sounds familar, that’s because it is:</p>
   <ul>
    <li data-md="">
     <p><a data-link-type="biblio" href="#biblio-p0258r2">[P0258r2]</a> introduced <code class="highlight"><span></span><span class="n">has_unique_object_representations</span></code>.</p>
    <li data-md="">
     <p><a data-link-type="biblio" href="#biblio-p0258r1">[p0258r1]</a> proposed <code class="highlight"><span></span><span class="n">is_contiguous_layout</span></code>.</p>
    <li data-md="">
     <p><a data-link-type="biblio" href="#biblio-p0029r0">[P0029r0]</a> proposed <code class="highlight"><span></span><span class="n">is_uniquely_represented</span></code>.</p>
    <li data-md="">
     <p><a data-link-type="biblio" href="#biblio-n3980">[N3980]</a> proposed <code class="highlight"><span></span><span class="n">is_contiguously_hashable</span></code>.</p>
    <li data-md="">
     <p><a data-link-type="biblio" href="#biblio-n3333">[N3333]</a> proposed <code class="highlight"><span></span><span class="n">is_contiguous_layout</span></code>.</p>
    <li data-md="">
     <p><a data-link-type="biblio" href="#biblio-n4130">[N4130]</a> discussed the padding bits issue, and wondered whether a <code class="highlight"><span></span><span class="n">has_padding_bits</span></code> trait was sensible. It was reviewed by SG1 in the 2014
Redmond meeting. Guidance was sought to also resolve <a data-link-type="biblio" href="#biblio-lwg2334">[LWG2334]</a> as well as
WG14 DR 431 while keeping C compatibility, but no conclusion was reached.</p>
   </ul>
   <p>The reader now also understands this paper’s Benjamin Button theme.</p>
   <h2 class="heading settled" data-level="2" id="word"><span class="secno">2. </span><span class="content">Proposed Wording</span><a class="self-link" href="#word"></a></h2>
   <h3 class="heading settled" data-level="2.1" id="word-meta-type-synop"><span class="secno">2.1. </span><span class="content">Header <code class="highlight"><span></span><span class="o">&lt;</span><span class="n">type_traits</span><span class="o">></span></code> synopsis [<strong>meta.type.synop</strong>]</span><a class="self-link" href="#word-meta-type-synop"></a></h3>
   <p>Add a new typetrait:</p>
<pre class="highlight"><ins><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> <span class="k">struct</span> <span class="n">has_padding_bits</span><span class="p">;</span></ins></pre>
   <p>and:</p>
<pre class="highlight"><ins><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> <span class="k">constexpr</span> <span class="kt">bool</span> <span class="n">has_padding_bits_v</span> <span class="o">=</span> <span class="n">has_padding_bits</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value</span><span class="p">;</span></ins></pre>
   <h3 class="heading settled" data-level="2.2" id="word-mta-unary-prop"><span class="secno">2.2. </span><span class="content">Type properties [<strong>meta.unary.prop</strong>]</span><a class="self-link" href="#word-mta-unary-prop"></a></h3>
   <p>Add to Table — Type property predicates:</p>
   <table>
    <thead>
     <tr>
      <th>Template
      <th>Condition
      <th>Preconditions
    <tbody>
     <tr>
      <td>᠁
      <td>᠁
      <td>᠁
     <tr>
      <td>
<pre class="highlight"><ins><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> <span class="k">struct</span> <span class="n">has_padding_bits</span><span class="p">;</span></ins></pre>
      <td>
       <ins> For an array type <code class="highlight"><span></span><span class="n">T</span></code>, the same result as <code class="highlight"><span></span><span class="n">has_padding_bits</span><span class="o">&lt;</span><span class="n">remove_all_extents_t</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>>::</span><span class="n">value</span></code>, otherwise <em>see below</em></ins>
      <td>
       <ins> <code class="highlight"><span></span><span class="n">T</span></code> shall be a complete type, (possibly cv-qualified) <code class="highlight"><span></span><span class="kt">void</span></code>, or an
      array of unknown bound. </ins>
     <tr>
      <td>᠁
      <td>᠁
      <td>᠁
   </table>
   <p>Add the following paragraph:</p>
   <blockquote>
    <ins>The predicate condition for a template specialization <code class="highlight"><span></span><span class="n">has_padding_bits</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">value</span></code> shall be satisfied if and only if:</ins>
    <ul>
     <li data-md="">
      <ins><code class="highlight"><span></span><span class="n">T</span></code> is trivially copyable, and</ins>
     <li data-md="">
      <ins>there exists at least one bit in the object representations of an
object of type <code class="highlight"><span></span><span class="n">T</span></code> which never participates in the value representation. </ins>
    </ul>
   </blockquote>
   <p>Or alternatively for the 2nd bullet:</p>
   <blockquote>
    <ul>
     <li data-md="">
      <ins>given two objects of type <code class="highlight"><span></span><span class="n">T</span></code>, for each bit in their object
representations that bit does not participate in their value
representations.</ins>
    </ul>
   </blockquote>
   <h3 class="heading settled" data-level="2.3" id="word-atomics-types-operations-req"><span class="secno">2.3. </span><span class="content">Requirements for operations on atomic types [<strong>atomics.types.operations.req</strong>]</span><a class="self-link" href="#word-atomics-types-operations-req"></a></h3>
   <p>Precede each of the following functions:</p>
   <ul>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">atomic_compare_exchange_weak</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">atomic_compare_exchange_strong</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">atomic_compare_exchange_weak_explicit</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">atomic_compare_exchange_strong_explicit</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">A</span><span class="o">::</span><span class="n">atomic_compare_exchange_weak</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">A</span><span class="o">::</span><span class="n">atomic_compare_exchange_strong</span></code></p>
   </ul>
   <p>
    With this concept: 
    <ins>requires <code class="highlight"><span></span><span class="n">has_padding_bits_v</span><span class="o">&lt;</span><span class="n">C</span><span class="o">></span></code></ins>
    .
   </p>
   <p>Update the following paragraph:</p>
   <blockquote>
    <p>
     <em>Requires:</em> The <code class="highlight"><span></span><span class="n">failure</span></code> argument shall not be <code class="highlight"><span></span><span class="n">memory_order_release</span></code> nor <code class="highlight"><span></span><span class="n">memory_order_acq_rel</span></code>. 
     <ins><code class="highlight"><span></span><span class="n">has_padding_bits_v</span><span class="o">&lt;</span><span class="n">C</span><span class="o">></span></code> shall be <code class="highlight"><span></span><span class="nb">false</span></code>.</ins>
    </p>
   </blockquote>
   <p>Update the following note:</p>
   <blockquote>
    <p>[ <em>Note:</em></p>
    <p>
     The <code class="highlight"><span></span><span class="n">memcpy</span></code> and <code class="highlight"><span></span><span class="n">memcmp</span></code> semantics of the compare-and-exchange operations may
  result in failed comparisons for values that compare equal with <code class="highlight"><span></span><span class="k">operator</span><span class="o">==</span></code> if the underlying type has 
     <del>padding bits</del>
     <ins>bits which don’t
  participate in active member’s value representation</ins>
     , trap bits, or
  alternate representations of the same value. Thus,
    </p>
    <del><code class="highlight"><span></span><span class="n">compare_exchange_strong</span></code></del>
    <ins>compare-and-exchange operations</ins>
     should be used with extreme care.
    <del> On the other hand, <code class="highlight"><span></span><span class="n">compare_exchange_weak</span></code> should converge rapidly.</del>
    <p><em>— end note</em> ]</p>
   </blockquote>
   <h3 class="heading settled" data-level="2.4" id="word-atomics-types-generic"><span class="secno">2.4. </span><span class="content">Atomic types [<strong>atomics.types.generic</strong>]</span><a class="self-link" href="#word-atomics-types-generic"></a></h3>
   <p>For all functions modified in <a href="#word-atomics-types-operations-req">§2.3 Requirements for operations on atomic types [atomics.types.operations.req]</a>, add the
same concepts to <code class="highlight"><span></span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code>'s <code class="highlight"><span></span><span class="n">compare_exchange_weak</span></code> and <code class="highlight"><span></span><span class="n">compare_exchange_strong</span></code> and all its specializations.</p>
   <h3 class="heading settled" data-level="2.5" id="word-atomics.syn"><span class="secno">2.5. </span><span class="content">Header <code class="highlight"><span></span><span class="o">&lt;</span><span class="n">atomic</span><span class="o">></span></code> synopsis [<strong>atomics.syn</strong>]</span><a class="self-link" href="#word-atomics.syn"></a></h3>
   <p>Add the same concept for all overloads of the following free function:</p>
   <ul>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">atomic_compare_exchange_weak</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">atomic_compare_exchange_strong</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">atomic_compare_exchange_weak_explicit</span></code></p>
    <li data-md="">
     <p><code class="highlight"><span></span><span class="n">atomic_compare_exchange_strong_explicit</span></code></p>
   </ul>
   <h2 class="heading settled" data-level="3" id="sample"><span class="secno">3. </span><span class="content">Sample Program</span><a class="self-link" href="#sample"></a></h2>
   <p>This program uses compare-and-exchange on a <code class="highlight"><span></span><span class="k">struct</span></code> which has padding bits. It
may loop infinitely, or not.</p>
<pre class="highlight"><span></span><span class="cp">#include</span> <span class="cpf">&lt;atomic></span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">&lt;cstring></span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">&lt;new></span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">&lt;stdio.h></span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">&lt;type_traits></span><span class="cp"></span>

<span class="k">struct</span> <span class="n">Padded</span> <span class="p">{</span>
  <span class="kt">char</span> <span class="n">c</span> <span class="o">=</span> <span class="mh">0xFF</span><span class="p">;</span>
  <span class="c1">// Padding here.</span>
  <span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mh">0xFEEDFACE</span><span class="p">;</span>
  <span class="n">Padded</span><span class="p">()</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">typedef</span> <span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="n">Padded</span><span class="o">></span> <span class="n">Atomic</span><span class="p">;</span>
<span class="k">typedef</span> <span class="n">std</span><span class="o">::</span><span class="n">aligned_storage</span><span class="o">&lt;</span><span class="k">sizeof</span><span class="p">(</span><span class="n">Atomic</span><span class="p">)</span><span class="o">>::</span><span class="n">type</span> <span class="n">Storage</span><span class="p">;</span>

<span class="kt">void</span> <span class="nf">peek</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span><span class="o">*</span> <span class="n">what</span><span class="p">,</span> <span class="kt">void</span> <span class="o">*</span><span class="n">into</span><span class="p">)</span> <span class="p">{</span>
  <span class="n">printf</span><span class="p">(</span><span class="s">"%16s %08x %08x</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">what</span><span class="p">,</span> <span class="o">*</span><span class="p">(</span><span class="kt">int</span><span class="o">*</span><span class="p">)</span><span class="n">into</span><span class="p">,</span> <span class="o">*</span><span class="p">(</span><span class="mi">1</span> <span class="o">+</span> <span class="p">(</span><span class="kt">int</span><span class="o">*</span><span class="p">)</span><span class="n">into</span><span class="p">));</span>
<span class="p">}</span>

<span class="n">Storage</span><span class="o">*</span> <span class="nf">create</span><span class="p">()</span> <span class="p">{</span>
  <span class="k">auto</span><span class="o">*</span> <span class="n">storage</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Storage</span><span class="p">();</span>
  <span class="n">std</span><span class="o">::</span><span class="n">memset</span><span class="p">(</span><span class="n">storage</span><span class="p">,</span> <span class="mh">0xBA</span><span class="p">,</span> <span class="k">sizeof</span><span class="p">(</span><span class="n">Storage</span><span class="p">));</span>
  <span class="k">asm</span> <span class="k">volatile</span><span class="p">(</span><span class="s">""</span><span class="o">:::</span><span class="s">"memory"</span><span class="p">);</span>
  <span class="n">peek</span><span class="p">(</span><span class="s">"storage"</span><span class="p">,</span> <span class="n">storage</span><span class="p">);</span>
  <span class="k">return</span> <span class="n">storage</span><span class="p">;</span>
<span class="p">}</span>

<span class="n">Atomic</span><span class="o">*</span> <span class="nf">change</span><span class="p">(</span><span class="n">Storage</span><span class="o">*</span> <span class="n">storage</span><span class="p">)</span> <span class="p">{</span>
  <span class="c1">// As if we used an allocator which reuses memory.</span>
  <span class="k">auto</span><span class="o">*</span> <span class="n">atomic</span> <span class="o">=</span> <span class="k">new</span><span class="p">(</span><span class="n">storage</span><span class="p">)</span> <span class="n">Atomic</span><span class="p">;</span>
  <span class="n">peek</span><span class="p">(</span><span class="s">"atomic placed"</span><span class="p">,</span> <span class="n">atomic</span><span class="p">);</span>
  <span class="n">std</span><span class="o">::</span><span class="n">atomic_init</span><span class="p">(</span><span class="n">atomic</span><span class="p">,</span> <span class="n">Padded</span><span class="p">());</span> <span class="c1">// Which bits go in?</span>
  <span class="n">peek</span><span class="p">(</span><span class="s">"atomic init"</span><span class="p">,</span> <span class="n">atomic</span><span class="p">);</span>
  <span class="k">return</span> <span class="n">atomic</span><span class="p">;</span>
<span class="p">}</span>

<span class="n">Padded</span> <span class="nf">infloop_maybe</span><span class="p">(</span><span class="n">Atomic</span><span class="o">*</span> <span class="n">atomic</span><span class="p">)</span> <span class="p">{</span>
  <span class="n">Padded</span> <span class="n">desired</span><span class="p">;</span>  <span class="c1">// Padding unknown.</span>
  <span class="n">Padded</span> <span class="n">expected</span><span class="p">;</span> <span class="c1">// Could be different.</span>
  <span class="n">peek</span><span class="p">(</span><span class="s">"desired before"</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">desired</span><span class="p">);</span>
  <span class="n">peek</span><span class="p">(</span><span class="s">"expected before"</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">expected</span><span class="p">);</span>
  <span class="n">peek</span><span class="p">(</span><span class="s">"atomic before"</span><span class="p">,</span> <span class="n">atomic</span><span class="p">);</span>
  <span class="k">while</span> <span class="p">(</span>
    <span class="o">!</span><span class="n">atomic</span><span class="o">-></span><span class="n">compare_exchange_strong</span><span class="p">(</span>
      <span class="n">expected</span><span class="p">,</span>
      <span class="n">desired</span> <span class="c1">// Padding bits added and removed here ˙ ͜ʟ˙</span>
  <span class="p">));</span>
  <span class="n">peek</span><span class="p">(</span><span class="s">"expected after"</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">expected</span><span class="p">);</span>
  <span class="n">peek</span><span class="p">(</span><span class="s">"atomic after"</span><span class="p">,</span> <span class="n">atomic</span><span class="p">);</span>
  <span class="k">return</span> <span class="n">expected</span><span class="p">;</span> <span class="c1">// Maybe changed here as well.</span>
<span class="p">}</span>

<span class="kt">int</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
  <span class="k">auto</span><span class="o">*</span> <span class="n">storage</span> <span class="o">=</span> <span class="n">create</span><span class="p">();</span>
  <span class="k">auto</span><span class="o">*</span> <span class="n">atomic</span> <span class="o">=</span> <span class="n">change</span><span class="p">(</span><span class="n">storage</span><span class="p">);</span>
  <span class="n">Padded</span> <span class="n">p</span> <span class="o">=</span> <span class="n">infloop_maybe</span><span class="p">(</span><span class="n">atomic</span><span class="p">);</span>
  <span class="n">peek</span><span class="p">(</span><span class="s">"main"</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">p</span><span class="p">);</span>
  <span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<span class="p">}</span></pre>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-lwg2334">[LWG2334]
   <dd>Daniel Krügler. <a href="http://cplusplus.github.io/LWG/lwg-active.html#2334">atomic's default constructor requires "uninitialized" state even for types with non-trivial default-constructor</a>. SG1. URL: <a href="http://cplusplus.github.io/LWG/lwg-active.html#2334">http://cplusplus.github.io/LWG/lwg-active.html#2334</a>
   <dt id="biblio-n3333">[N3333]
   <dd>J. Yasskin, C. Carruth. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3333.html">Hashing User-Defined Types in C++1y</a>. 13 January 2012. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3333.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3333.html</a>
   <dt id="biblio-n3980">[N3980]
   <dd>H. Hinnant, V. Falco, J. Byteway. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3980.html">Types don't know #</a>. 24 May 2014. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3980.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3980.html</a>
   <dt id="biblio-n3986">[N3986]
   <dd>S. Davalle, D. Gutson, A. Bustamante. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3986.pdf">Adding Standard support to avoid padding within structures</a>. 25 April 2014. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3986.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3986.pdf</a>
   <dt id="biblio-n4130">[N4130]
   <dd>JF Bastien, O. Giroux. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4130.pdf">Pad Thy Atomics</a>. 1 September 2014. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4130.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4130.pdf</a>
   <dt id="biblio-p0020r3">[P0020r3]
   <dd>H. Carter Edwards, Hans Boehm, Olivier Giroux, JF Bastien, James Reus. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0020r3.html">Floating Point Atomic View</a>. 14 October 2016. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0020r3.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0020r3.html</a>
   <dt id="biblio-p0029r0">[P0029r0]
   <dd>Geoff Romer, Chandler Carruth. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0029r0.html">A Unified Proposal for Composable Hashing</a>. 21 September 2015. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0029r0.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0029r0.html</a>
   <dt id="biblio-p0258r1">[P0258R1]
   <dd>Michael Spencer. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0258r1.html">is_contiguous_layout</a>. 5 March 2016. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0258r1.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0258r1.html</a>
   <dt id="biblio-p0258r2">[P0258r2]
   <dd>Michael Spencer. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0258r2.html">has_unique_object_representations - wording</a>. 24 June 2016. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0258r2.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0258r2.html</a>
  </dl>